Amgen will pay $125 a share for Onyx’s outstanding stock, the companies said in a statement yesterday. Net of estimated Onyx cash, the deal is worth $9.7 billion. Onyx’s Kyprolis, approved last year for a rare blood cancer, may spur more than $3 billion in revenue by 2021, according to analyst estimates compiled by Bloomberg. South San Francisco-based Onyx is now studying the medicine in an expanded group of patients.

The accord mirrors recent deals in which drugmakers bought companies with one or two promising products, rather than attempting large mergers that come with whole pipelines or offer business synergies. Kyprolis fills a hole for Amgen in a product line that largely contains drugs to support rather than treat cancer patients, at a time when the oncology market is growing increasingly important as the U.S. population ages.

“It’s a steal for Amgen,” said Ori Hershkovitz, a partner at Sphera Funds Management Ltd., a Tel Aviv-based health-care hedge fund that holds both Onyx and Amgen shares. “It has secured its long-term growth. Securing your long-term growth for $10 billion, I think you made a good acquisition.”

Next Transaction

The next buyout within the drug industry may involve Alexion Pharmaceuticals Inc. (ALXN), another single-product company whose blood-disease treatment Soliris generated $1.1 billion in sales last year as one of the world’s most expensive medicines. Alexion engaged Goldman Sachs Group Inc. as an adviser as it prepared for a possible takeover offer from Roche Holding AG, people with knowledge of the matter said last month.

Amgen’s deal for Onyx follows similar agreements featuring a small number of already-marketed or late-stage experimental drugs.

Last year, for example, Bristol-Myers Squibb Co. bought Inhibitex Inc. and Amylin Pharmaceuticals to gain individual treatments for diabetes and hepatitis C. The chief executives for both Pfizer Inc. and Merck & Co. have said this year that they prefer similar “bolt-ons,” rather than mergers that come with a slate of products.

‘Unique Opportunity’

While Thousand Oaks, California-based Amgen has gained from its dominance of the anemia market, the company has been seeking products to expand its product portfolio in cancer. Eight late-stage medicines are in development that will generate clinical trial data over the next three years, Chief Executive Officer Robert Bradway said in February.

Bradway said today Amgen wanted to get in early to tap an expected surge in the blood-cancer drug’s growth. “It’s at a point where we feel we can help maximize the full potential of the product,” he said on a conference call discussing the deal.

The price for Onyx is 44 percent higher than the stock’s $86.82 value on June 28, before the discussions with Amgen were disclosed. The deal will be financed using $8.1 billion in bank loans and cash it has in the U.S., the companies said in the statement. Approved by both boards, the transaction should close in the fourth quarter, they said.

The price “leaves some net present value on the table for Amgen shareholders, but still allows Onyx shareholders to enjoy a substantial premium,” said Mark Schoenebaum, an analyst with International Strategy & Investment Group in New York. “In addition, Onyx management can exit after having created substantial value in a relatively short period of time.”

Drug Data

At the same time, the deal may signal that prices may be dropping for biotechnology generally, Schoenebaum said. Amgen and Onyx agreed to the $125-a-share price after an earlier offer of $130 fell apart, people familiar with the negotiations said previously. The parties couldn’t resolve a dispute about Onyx drug data, said one of the people. And other drugmakers didn’t step in with higher bids.

The information Amgen was able to see was sufficient, Bradway said. “We reviewed the data, that are available to us, and our confidence is reflected in the price that we moved forward with the transaction with,” he said today.

Onyx’s shares soared after the company announced Amgen’s original $120-a-share offer at the end of June. That signaled some investors expected competing offers. AstraZeneca Plc, Pfizer and Novartis AG expressed interest, Bloomberg reported in July.

Schoenebaum questioned whether the lowered price meant companies were becoming more skeptical of valuations. “Is there price sensitivity out there?” he asked in an e-mail. “Why didn’t it go for more? The Onyx analysts all said originally this would go for $140 or $150.”

Amgen’s Deals

The Onyx acquisition will be Amgen’s largest after its 2001 purchase of Immunex Corp. for $16.8 billion, data compiled by Bloomberg show. Amgen’s next-largest deal was its 2005 acquisition of Abgenix Inc. for $2.21 billion.

In addition to Kyprolis, Onyx sells Nexavar for liver and kidney cancer in partnership with Germany’s Bayer AG. Onyx generated $362 million in 2012 revenue, with 80 percent coming from Nexavar and the stomach-cancer treatment Stivarga. The company gets a 20 percent royalty on Stivarga from Bayer, which has said it expects the medicine to be a bestseller.

Amgen’s top product is Enbrel, a rheumatoid arthritis treatment that sold $4.24 billion in 2012. Neulasta, an immune system booster for chemotherapy patients, sold $4.09 billion. Aranesp, which helps raise kidney failure and cancer patients’ red blood cell counts, sold $2.04 billion, followed by Epogen, which does the same, at $1.94 billion.

Coles Impact

Onyx CEO N. Anthony Coles joined the company in March 2008. Since taking over, the shares have increased fourfold.

The purchase is the third-largest acquisition of a biotechnology company in the past three years, according to data compiled by Bloomberg. Since August 2010, 62 biotechnology deals valued at $50 million or more were announced, with an average disclosed price of $1.29 billion and an average premium of 55 percent, the data show.

Sanofi’s 2011 purchase of rare-disease drugmaker Genzyme Corp. for $20.1 billion was the largest, followed by Gilead Sciences Inc.’s purchase of Pharmasset Inc. for $10.6 billion.

Lazard Ltd. and Bank of America acted as financial advisers to Amgen, and Sullivan & Cromwell were legal advisers. Centerview Partners were financial advisers to Onyx, and Goodwin Procter LLP were legal advisers.